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Sewage Treatment Plants: Getting the Size Right

When you don’t have mains drainage, installing the right size Sewage Treatment Plant is critical.

Throughout this website our Sewage Treatment Plants are categorised by population (P) size to be served, and this page will help you calculate the right size system for a domestic property or group of properties.

Single Domestic Dwelling up to and including 3 bedrooms:

·         Sewage Treatment Plant should be installed for minimum 5 population (P).

·         So minimum single domestic dwelling = 5 P Sewage Treatment Plant.

·         Add 1 P for each additional bedroom.

·         So 4 bedroom house = minimum 6P Sewage Treatment Plant and so on.

Groups of smaller 1-2 bedroom houses or flats:

·        For groups of small 1 and 2 bedroom flats, a total P load needs to be calculated specifically for each group taking into consideration the maximum possible occupancy and flow.

For groups of larger houses:

·         Add the P values calculated for each house to determine overall system size.

·         E.g. 2 houses (3 & 4 Bedrooms respectively) = 11 P Sewage Treatment Plant.

Where the total P load for a group of dwellings exceeds 12:

·         Reduction can be made to adjust for daily flow fluctuations.

·         Always round up not down.

Where P load total is 13-25:

·         Multiply total P load by 0.9 to provide adjusted P value.

·         E.g. Four 4 bedroom houses, total P value 4 x 6 = 24

·         24 x 0.9 = 21.6 rounded up = adjusted P value of 22.

Where P value total is 26-50:

·         Multiply total P load by 0.8 to provide adjusted P value.

·         E.g. Four 3 bedroom houses and three 4 bedroom houses,  total P value = 38 

·         38 x 0.8 = 30.4 rounded up = adjusted P value of 31.

These are minimum recommended population (P) loads and they should not be modified downwards.

Upward modification may be necessary because of particular characteristics of each property or groups of properties.

The above assessments of population (P) should be used for both new and existing properties,

Note that larger and more luxurious houses tend to have greater loads and increased water consumption with variability.

Holiday homes tend to have higher occupancies since lounges frequently act as bedrooms. Holiday lets and second homes may be used intermittently.

Check for unusual water uses such as spa baths, home brewing or home photo processing.

Waste disposal units increase biological load.

Laundry chemicals and toxic substances will affect the performance of a sewage treatment plant. 

This information is based on our understanding of the code of practice produced by the British Water Packager Sewage Treatment Plant Focus Group.

This understanding includes the fact that the Environment Agency, Environment and Heritage Service (Northern Ireland) and Scottish Environment Protection Agency support the use of this code of practice, however please note that these Agencies do not specifically endorse any particular manufacturer's product.

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